Lithium ion secondary batteries have advantages such as high energy density, small self-discharge, excellent long-term reliability and the like, and therefore they have been put into practical use in notebook-type personal computers and mobile phones. More recently, the development of the high performance lithium ion secondary battery excellent in cycle characteristics and further improved in capacity and energy density is demanded due to, in addition to the trend of high functionality of electronic equipment, the expansion of market of motor driven vehicles such as electric vehicles and hybrid vehicles and the acceleration of the development of domestic and industrial power storage systems.
Attention has been drawn to metallic active materials such as silicon, tin and alloys and metal oxides thereof as a negative electrode active material which provides a high capacity lithium ion secondary battery. These metallic negative electrode active materials provide a high capacity, but the expansion and contraction of the active materials are large when lithium ions are absorbed and released. For this reason, as the negative electrode binder containing such a metal-based negative electrode active material having a large expansion and contraction during charge and discharge, it is preferable to select one having a strong binding force capable of withstanding the volume change of the active material.
Patent Document 1 discloses that polyacrylic acid is good as a binder used for a negative electrode containing an active material, such as silicon, tin, or the like, having a large volume change during charge and discharge. This is because the polyacrylic acid has a large number of carboxyl groups as a functional group, providing strong binding force, and is chemically stable. Furthermore, Patent Document 1 discloses that an ethylene-acrylic acid copolymer can be used as a polyacrylic acid binder to improve the flexibility of the polyacrylic acid and to suppress destruction of the binding structure of the active materials due to the expansion and contraction. This suppression of the destruction of the binding structure of the active materials improves the charge and discharge cycle characteristics of the lithium ion secondary battery.